1. Field of the Invention
This invention relates to a rotational polygon mirror used as a reflecting mirror in a copying apparatus, a facsimile apparatus, a laser beam printer or the like, and more particularly to a metallic rotational polygon mirror having a high reflection factor and excellent durability.
2. Description of the Prior Art
Some copying apparatuses, facsimile apparatuses, laser beam printers or the like use a rotational polygon mirror having a reflecting surface of high accuracy. Such a rotational polygon mirror is generally made by diamond-cutting a soft metal blank such as Al or Cu, and it is well known that these metal materials are susceptible to chemical change by the outside atmosphere. For example, Al creates an Al.sub.2 O.sub.3 layer on the surface thereof by oxidation and reduces its reflection factor. Also, Cu is liable to react chemically and cause oxidation and corrosion. Further, these metals are very soft and therefore are remarkably susceptible to surface damage from mechanical contact, such as wiping the cut mirror surface. Also, where a semiconductor laser of near-infrared light is used as as the light source, the reflection factor of the polygon mirror need be high from the necessity of enhancing the energy of reflected light relative to the high-speed rotation of the rotational polygon mirror. As a countermeasure for these problems, it has heretofore been practised to form on the metallic mirror surface a single-layer or multi-layer film of dielectric material as a protective film or a reflection increasing film by means such as vacuum evaporation. For example, the rotational polygon mirror shown in Japanese Laid-open Patent Application No. 22003/1984 was made by super-precision-working an aluminum alloy to form a reflecting surface, and thereafter providing an aluminum coating film, an SiO.sub.2 coating film and a TiO.sub.2 coating film in sucession on the reflecting surface, and the protection of the aluminum coating film surface and the increased reflection are accomplished by a thin dielectric material film formed of SiO.sub.2 and TiO.sub.2.
However, where the reflecting surface is formed of aluminum or an alloy thereof, even if the increased reflection is accomplished by a thin dielectric material film, the reflection factor of the polygon mirror has been limited because the reflection factor of aluminum itself is low. Accordingly, it would occur to mind to provide on an aluminum blank a thin metal film having a higher reflection factor than aluminum to thereby improve the reflection factor, but in this case, corrosion would occur due to the difference in ionization tendency between the aluminum blank and the thin metal film, or a problem would arise in the film strength and the intimate contact property of the thin metal film.